1. Field of the Invention
The present invention relates to high-speed modem telecommunications. More specifically, the present invention discloses a method for efficiently calculating the step-sizes for a frequency domain equalizer in a discrete-multitone communications system.
2. Description of the Prior Art
Frequency domain equalization (FEQ) for discrete multi-tone (DMT) communications systems has involved separate processes for gain equalization (GE) and phase equalization (PE). In digital subscriber line (DSL) systems, FEQ must be performed when the communications are occurring, since each subscriber loop, typically twisted pair line, has individual characteristics for attenuation and frequency response due to bridge taps, outside interference, length of the line, and other factors. The frequency responses of several real-world ADSL loops are shown in chart form in FIG. 2. FEQ is performed during the training phase as the DSL modems on each end of the twisted pair line negotiate to determine the subchannel signal to noise ratio (SNR) and bit allocation.
Please refer to FIG. 7, which shows a block diagram of a typical DMT-based communications system. A time-domain equalizer TEQ shortens the channel dispersion to minimize intersymbol interference (ISI) caused by channel distortions such as loop length, gauge change, and bridge taps. Although TEQ can minimize ISI in the time domain, phase rotation and amplitude attenuation in the frequency domain still exist on each subchannel. To overcome these distortions, an FEQ is used to compensate for the phase and amplitude distortions on each subchannel.
However, existing methods for calculating FEQ are complex and use significant processing resources, either in software computing cycles or in specialized hardware such as high performance DSPs. One such prior art frequency domain equalization method is disclosed in U.S. Pat. No. 6,389,062 by Wu. The method uses two equalizers, a phase equalizer (PE) and a gain equalizer (GE), in order to implement frequency domain equalization. A block diagram of this prior art method is shown in FIG. 1. The FEQ input Yk is processed by gain equalizer GE and phase equalizer PE.
In addition, the step-size used by the FEQ for training is a constant chosen through off-line simulation at the time of hardware design, with many uncertain issues left unresolved, and thus, the signal to noise ratio (SNR) under real-world conditions cannot be compensated for, resulting in larger net errors in phase and amplitude corrections, reducing real-world efficacy and reliability during negotiation.
Therefore there is need for improvement in frequency domain equalization for DMT-based communications systems to overcome the uncertainty of the prior art.